What is the difference between a functional dependency and a multivalued dependency?
I am reading both definitions, but it's hard to differentiate. It appears a functional dependency is when one value is based on a similar concept as another value, and if you know one, you probably also know the other. I don't get how this is different from a multivalued dependency, which seems to have a similar definition.
Can someone point out to me where the two definitions contrast each other?
functional dependency: Given a relation R, a set of attributes X in R is said to functionally determine another attribute Y, also in R, (written X → Y) iff each X value is associated with precisely one Y value.
in other words, Y is dependent on X
in practical terms: give me a value of X, and i can tell you the value of Y that corresponds to it
SELECT Y FROM daTable WHERE X = 42
functional dependency plays an important part in determining whether a table is in 2NF
Rudy, I think I may possibly have my terminlogy mixed up. I think I may be talking about a transitive dependancies. The way that I understand 3NF, all of the transitive dependancies have to be removed from my table in order to achive 3NF.
Is it possible that I am confusing the terms? The reason I think I am confusing these two terms is after you said that you want FD in a table, it made me think of 2NF where everything must be dependant on the whole key, and that is a good thing.
Ok. cool. In post #3, I was actually talking about finding a transitive dependancy. Rudy, would you please tell me if what I am asking in #3 still applies? Would I still exclude outside sources when trying to resolve a TD?
gotta be honest, i didn't understand that vin/plate stuff in post #3, but generally speaking, you would do modelling based not on what data is available in the external world, but on the data in your application